Youth perfectionism levels have increased significantly over the last decades. Given the dominance of picture-perfect content, social media are often designated as contributors of this rise. Accordingly, this study examined how exposure to positive social media content might increase adolescents’ perfectionistic dispositions and vice versa. Moreover, it was explored whether upward social comparison and adolescents’ individualism values as well as country play, respectively, a mediating and moderating role. A three-wave panel study was conducted among 1,697 Belgian, French, and Slovenian adolescents (Mage = 15.14, SDage = 1.78, 57.2% girls). At the between-person level, the results showed that higher levels of exposure to positive content are related to higher levels of perfectionism. Higher upward social comparison levels are related to higher levels of exposure to positive content and of socially prescribed perfectionism. A more complex pattern emerged at the within-person level with different results depending on the selected time intervals, countries, and adolescents’ individualistic values.